1. Field of the Invention
The present invention refers to rubberized fabrics for tires having zero-degree reinforcing elements and to tires incorporating such fabrics.
2. Description of the Related Art
As is known, a tire includes at least three fundamental components, the carcass, the tread band, and the reinforcing belt between the tread band and the carcass. The carcass, usually at least one ply, is turned out at both ends around a pair of bead cores. Together, the bead cores, the ends of the carcass, and whatever rubberized filler is added between the bead cores and the ends of the carcass cooperate to form the beads on either side of the tire.
When in use, a tire is placed on a wheel rim, which has two seats axially displaced from one another. The two beads on either side of the tire rest on the two rim seats. Each of the rim seats terminates in an end flange, which has an outermost diameter greater than the diameter of the tire beads, which prevents the beads from slipping off of the wheel rim once the tire is installed on the rim.
The belts of a conventional tire generally consist of at least three rubberized fabrics. The first two fabrics comprise fine cords that crisscross each other and are both angled with respect to the equatorial plane of the tire. The third fabric is external radial belt made from fine cords of heat-shrinkable material including synthetic fibers, oriented at 0° with respect to the equatorial plane of the tire, such as commonly nylon. In one common embodiment, the third layer consists of nylon cords 0.39 mm in diameter embedded in a 0.7-mm-thick rubberized fabric.
To form the tread pattern on a tire, an uncured or “green” tire is placed in a mold, which carries on its interior surface the pattern for the tread of the tire. During the tread forming step in the manufacturing process, the mold is pushed into the tire to imprint the tread pattern onto the tire. At the same time, the tire is inflated, causing the layers with crisscrossed cords to expand toward the interior surface of the mold. This expansion helps to push the tread band againsto the surface of the mold so that it can accept the tread pattern from the mold. As the tire expands when inflated, the crisscrossed cords are pushed outwardly, diminishing the angle of inclination of the cords with respect to one another. The third belt layer, with its heat-shrinkable synthetic fiber cords, exerts a force on the layers beneath it to limit their outward movement during the tread-forming stage.
Not only does the third belt layer serve a purpose in the manufacture of a tire, it also is important to the operation of a tire when the tire is mounted on a rim. The third belt layer helps to counteract the outward expansion of the underlying layers, which is caused by the large centrifugal forces that act on the belts at high speeds.
The synthetic fiber cords, however, have at least one disadvantage. They are known to temporarily deform in a tire in a phenomenon known as “flatspotting.” When the vehicle is stopped, the entire weight of the car rests on one spot on each of the tires. This causes a flattening of each tire in the footprint or imprint area where the tire contacts the ground. Because the synthetic fibers are prone to creep under stress, they distort in the imprint area. Even after the vehicle begins to move and the tire rotates, the flattened region persists in the imprint area for a prolonged period of time. Such a phenomenon is typical of all synthetic materials. The phenomenon varies from material to material depending on the viscoelastomeric characteristics of the particular synthetic fiber in question. Consequently, at least for a certain period after the tire rotates following flatspotting, the temporary deformation of the synthetic cords generates a noise effect and uncomfortable behaviour.
To avoid this phenomenon, it is known to incorporate zero-degree metallic cords (oriented at zero degrees with respect to the equatorial plane of the tire) in the third belt rather than cords of synthetic material. The metallic cords are sufficiently rigid to resist deformation when the vehicle is stopped. Tires made according to this teaching do not exhibit the phenomenon of “flatspotting” because they incorporate metallic cords rather than cords made from a viscoelastic material.
When the third belt layer is constructed with metallic cords, it is known to use cords made of some strands twisted together of the so called “lang lay type” that provides cords with high elongation prior to reaching their breaking point, and, because of this, the cords are also known in the prior art as “HE” (high-elongation) cords. In such an embodiment, the metallic cords act like a spring wire, which is evident from studying a typical stress-deformation diagram for these materials.
The first segment of the stress-deformation diagram for metallic cords is identified by a small or weak slope with respect to the abscissa. This first segment of the stress-deformation diagram is useful because it can be used to predict the behavioral characteristics of the metal during the tread band forming stage where the material exhibits high elongation at low loads. The next, strongly sloped segment of the stress-deformation diagram is useful for determining the behavioral characteristics of the metal during operation of the tire, where the material exhibits only slight elongation under a high load.
The overall diameter of metallic cords suitable for this purpose may be 0.7 mm in a zero-degree fabric used in the manufacturing of large tires. However, such a cord size is too large to be compatible with the dimensions required for a belt fabric in an automobile tire.
Belt structures with metal cords made from shape-memory materials are also known in the art, e.g., U.S. Pat. No. 5,242,002 and Japanese Patent Application JP 4362401. In U.S. Pat. No. 5,242,002, a tire is described with belts having cords symmetrically inclined with respect to the equatorial plane of the tire. The cords are formed by helically winding several wires together. At least one of the wires in the cord is made from a shape-memory material. The shape-memory wire, before being cabled with the other wires, undergoes a heat treatment at a predetermined temperature while it is in a particular configuration (for example undulated) and is subsequently deformed into a linear configuration below the temperature of the heat treatment; accordingly, said wire recover the undulated configuration above the heat treatment temperature.
Each time the temperature of the tread band increases at high speeds, the temperature of the belt exceeds that of the heat treatment of the shape-memory wire, and the wire tends to take on the undulated shape. However, since the shape-memory wire is corded with the other wires, the shape-memory wire cannot deform but, instead, is subject to tension. As a result, in the shape-memory wire, a stress is established, the effect of which is to increase the rigidity of the belt and, accordingly, avoid an increase in the diameter of the tire caused by centrifugal forces.
Japanese Patent Application JP 4362401 discloses a tire with an outermost belt having an outermost layer comprising a shape-memory expansion element, preferably a spring wire element made from a Ni—Ti alloy. The spring wire element is wound at zero degrees over the underlying layers of the belt. The shape-memory element is designed to contract in the peripheral direction of the tire when the wire is heated during high speed use. In this way, at high speeds, the tire becomes more rigid and the phenomenon of tire expansion is controlled. On the other hand, at low speeds, such as those encountered under normal traveling conditions, the shape-memory element returns to and maintains its original shape. The Japanese application describes wires from 0.25 to 0.5 mm in diameter. Finally, the Japanese application discloses that it is not necessary for the shape-memory expansion element in the tire to be spring wire shaped, but that it can be shaped as a belt or cord, for example.